In numerous electrical power generating systems there is a need to eliminate undesired AC ripple signals coexisting with a desired signal on a bus. Typically these undesired AC ripple signals are generated by known system components such as power sources or are results of reflections caused by unbalanced loads. In either case, these undesired AC ripple signals are reflected throughout the circuitry of the system. This is particularly true in variable speed, constant frequency (VSCF) power generating systems, such as those typically used in aircraft.
In an ideal VSCF system, a prime mover turns a generator which typically produces a three-phase electrical signal, the instantaneous frequency of which is dependent upon the speed of the prime mover. A rectifier bridge then rectifies this three-phase, variable frequency signal to produce a substantially DC signal on a bus. Next, an invertor inverts the DC signal in response to a control signal to produce a three-phase AC signal. An output filter filters this three-phase AC signal to eliminate substantially all but the desired fundamental frequency thereby producing a constant frequency, three-phase signal at the power system output.
However, due to the complex nature of the loads typically connected to VSCF power systems, particularly those found in aircraft design, and due to the fact that these loads frequently change as a result of operator control actions, it is impossible to know the precise load factor connected to the power system at any particular time. This generally results in the loads being unbalanced with respect to the power generator for a majority of the loads.
Because of the unbalanced nature of the loads, AC ripple signals having frequencies corresponding to harmonics of the signals supplied by the invertor are reflected back through the power system and specifically onto the DC bus. Because of these AC ripple signals appearing on the DC bus, the invertor no longer receives a pure DC signal at its input, but instead receives a DC signal containing an AC ripple component. This AC ripple component causes the invertor to produce additional harmonics at its output.
One typical method of correcting for these AC ripple effects is through a feedback loop which samples the output of the power system to determine when phase and frequency errors appear. This feedback loop compares the system output with a reference signal to produce an error signal. This error signal then drives circuitry to compensate for the output distortion. One problem with such a feedback configuration is that distortion must appear on the power system output line before compensation for such distortion can occur. In other words, the compensation circuitry will always lag the distortion occurring at the power system output thus allowing some distortion to be transmitted to the load. Examples of compensation networks are shown in Kirchberg et al., U.S. Pat. No. 4,977,492 and Kirchberg et al., U.S. Pat. No. 4,994,956, both of which describe a feedback loop connected to the output of a VSCF power system for modulating the switching of the invertor so as to eliminate distortions produced by the invertor. One drawback of these systems is the high amount of computing power necessary for their operation.
Another well known method of eliminating ripple appearing on a bus is by connecting passive notch filters to the bus, with such filters being tuned to the specific frequencies that are expected to appear on the bus. However, because of the high power which generally exists on the DC bus in a VSCF power system, these passive notch filters are typically very heavy, require a substantial amount of space, and are intolerant to frequency shift.
A further method of eliminating undesired ripple on a bus is with the use of an active notch filter capable of changing its passband frequency over a limited range. Mueller, U.S. Pat. No. 3,628,057 shows one implementation of such a technique. Mueller describes a filter connected to a bus for eliminating undesired frequency components from the bus, with a correction circuit for controlling the center passband frequency of the filter so as to compensate for errors introduced by the filter circuitry or for shifts in the power source frequency.
Takeda et al., U.S. Pat. No. 4,812,669 shows the combined use of a passive filter and an active notch filter to eliminate AC ripple from a bus. The passive filter consists of a combination of inductors and capacitors while the active filter comprises a PWM controller which drives the operation of a network of transistors and diodes operating so as to eliminate undesired components of a signal existing on a bus.